The present invention relates to a method for the anodic bonding together of glass and semiconducting material.
Anodic bonding is a relatively low-temperature process in which semiconducting materials are hermetically sealed to glass for use in microelectro-mechanical sensor (MEMS) devices. Anodic bonding produces strong hermetic seals which protect the device from premature failure due to interactions with corrosive environments, externally applied mechanical stresses, and fluctuations in internal pressure. As a result, MEMS devices are able to sense different magnitudes of the same stimuli without compromising the original response characteristics to which the integrated circuitry was normalized.
Several types of commercial glasses have been used to anodically bond to silicon or other metals, such as Pyrex 7740/7070/1729/9626, Tempax and Hoya SD-2 glass. In the past, methods to reduce the anodic bonding temperature have focused on either the development of new glasses or the application of a thin glass film interlayer.
A recent study utilized a lithium aluminosilicate-xcex2 quartz glass ceramic to lower the anodic bonding temperature to silicon. Successful bonds were reported at temperatures as low as 160xc2x0 C., however, the reported strength was only xcx9c2.5 MPa. Typical anodic bond strengths for Pyrex 7740 range between 10-20 MPa. Successful integration of this glass ceramic into MEMS devices has not been reported. Another study focused on the development of a lithium borosilicate glass in order to lower anodic bonding temperatures to 200xc2x0 C. when a voltage of 2000VDC was applied. V. Baier, et al xe2x80x9cAnodic Bonding at Low Temperatures using Laser Microstructurable Li-Doped Glass,xe2x80x9d Fourth International Symposium on Semiconductor Wafer Bonding: Science, Technology, and Applications, Eds. U. Goselle, et al Electrochemical Society: Pennington (1998) 222-228. The authors did not report bond strength nor has this glass been used as a cover by the MEMS industry. It is assumed that the cost of these glasses makes them unlikely candidates for replacement of Pyrex 7740.
Intermediate thin films of sputtered or evaporated sodium borosilicate glass on silicon substrates have been used to lower anodic bonding temperatures. However, with this method, additional processing steps are required before bonding can proceed. For instance, the application of the thin film is slow and an annealing step ( greater than 300xc2x0 C.) is necessary in order to reduce the residual stresses in the film. Unfortunately, in many of these studies, bond strengths were not reported. One study, however, was able to successfully bond silicon-to-silicon using evaporated Pyrex 7740 at temperatures as low as 225xc2x0 C. Reported bond strength was relatively high at 30 MPa, but the bonding procedure lasted between 2-3 hours.
Typically Pyrex 7740 is used as the glass cover, because it is commercially available, relatively inexpensive, easy to bond, has a similar coefficient of thermal expansion to that of silicon and acts as an insulator so only low levels of parasitic capacitance may be introduced into the package. With the development of new glasses, the same criteria must be met. However, the temperatures currently used for this bonding technique typically range between 300-450xc2x0 C. for Pyrex. At temperatures above 280xc2x0 C., thermal stresses are introduced into the package due to the thermal expansion mismatch between silicon and Pyrex causing the package to warp or bow. With the introduction of thermal stresses into extremely sensitive MEMS devices, the reliability and lifetime of the product is reduced. Also of concern is the fact that at these elevated temperatures, temperature-sensitive electronics and low melting point materials integrated into the MEMS device are susceptible to damage.
Further discussion of the prior art of anodic bonding can be found in U.S. Pat. Nos. 3,417,459 and 3,397,278 to D. I. Pomerantz, et al.
It is therefore an object of the present invention to provide an improved low temperature method for the anodic bonding of glass to semiconducting material, especially silicon.
This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying schematic drawings, in which:
FIG. 1 shows an ion exchange apparatus for the surface modification of glass pursuant to the method of the present invention;
FIG. 2 illustrates one exemplary embodiment of an anodic bonding apparatus; and
FIG. 3 is a graph in which temperature is plotted against time to illustrate the anodic bond of untreated Pyrex wafers.